#include <stdio.h>
#define M 10
int main()
{
#if M == 0
  printf("M is defined\n");
#elif M == 1
  printf("M is defined\n");
#else
  printf("M is not defined\n");
#endif
  printf("\n");
  // 检测是否定义了M ↓
#if !defined(M)
  printf("M is not defined\n");
#else
  printf("M is defined\n");
#endif
  printf("\n");
  // 检测是否定义了M ↓
#ifdef M
  printf("M is defined\n");
#else
  printf("M is not defined\n");
#endif
  return 0;
}